Capillaries streaming potential

Streaming potentials, like other electrokinetic effects, are difficult to measure reproducibly. One means involves forcing a liquid under pressure through a porous plug or capillary and measuring E by means of electrodes in the solution on either side [6, 23, 71-73]. 

Streaming potential measurements are to be made using a glass capillary tube and a particular electrolyte solution, for example, O.OIM sodium acetate in water. Discuss whether the streaming potential should or should not vary appreciably with temperature. 

FIGURE 31.1 Schematic design of cells for studying electroosmosis (a) and streaming potentials (b), the velocity of electroosmotic transport can be measured in terms of the rate of displacement of the meniscus in the capillary tube (in the right-hand part of the cell). 

The electrokinetic processes can actually be observed only when one of the phases is highly disperse (i.e., with electrolyte in the fine capillaries of a porous solid in the cases of electroosmosis and streaming potentials), with finely divided particles in the cases of electrophoresis and sedimentation potentials (we are concerned here with degrees of dispersion where the particles retain the properties of an individual phase, not of particles molecularly dispersed, such as individual molecules or ions). These processes are of great importance in particular for colloidal systems. 

There are four related electrokinetic phenomena which are generally defined as follows electrophoresis— the movement of a charged surface (i.e., suspended particle) relative to a stationary liquid induced by an applied electrical field, sedimentation potential— the electric field which is crested when charged particles move relative to a stationary liquid, eleetroosmosis—the movement of a liquid relative to a stationary charged surface (i.e., capillary wall), and streaming potential—the electric field which is created when liquid is made to flow relative to a stationary charged surface. The effects summarized by Eq. (20-23) form the basis of these electrokinetic phenomena. 

Figure 12.7b is a sketch of an apparatus that may be used to measure streaming potential. As was the case with electroosmosis, the capillary can be replaced by a plug of powdered material between perforated electrodes. An applied pressure difference p across the capillary causes the solution to flow through the capillary, thereby tangentially displacing the part of the double layer in the mobile phase from the stationary part. 

If both sides of Equation (63) are multiplied by the length of the capillary (, the potential difference between the measuring electrodes —the streaming potential Eslr—is obtained ... 

Two conditions must be met to justify comparisons between f values determined by different electrokinetic measurements (a) the effects of relaxation and surface conductivity must be either negligible or taken into account and (b) the surface of shear must divide comparable double layers in all cases being compared. This second limitation is really no problem when electroosmosis and streaming potential are compared since, in principle, the same capillary can be used for both experiments. However, obtaining a capillary and a migrating particle wiih identical surfaces may not be as readily accomplished. One means by which particles and capillaries may be compared is to coat both with a layer of adsorbed protein. It is an experimental fact that this procedure levels off differences between substrates The surface characteristics of each are totally determined by the adsorbed protein. This technique also permits the use of microelectrophoresis for proteins since adsorbed and dissolved proteins have been shown to have nearly identical mobilities. 

In their study of the effects of hydrolyzable cations on electro kinetic phenomena (see Problem 4), James and Healy compared the electrophoretic behavior of colloidal silica with the streaming potential through a silica capillary. In both sets of experiments the solution was 10-3 M KN03 and 10 4 M Co(N03)2. The following results were obtained ... 

The classical equations relating streaming current or streaming potential to zeta potential are derived for the case of a single circular capillary as follows. 

A is the conductivity of the solution. A streaming potential is established by a confined solution flowing under pressure through small-diameter pores and capillaries. It is believed that the confining walls, typically glass, become charged with OH-, thereby initiating the potential. 

If a pressure difference, AP, is applied between the extremes of a capillary, then a potential difference is created, called the streaming potential ... 

For endosmose a potential difference is applied between the two ends of a capillary tube, or the two sides of a porous plug or membrane, and the rate of motion, or the pressure required to prevent motion, of the liquid, is noted. For streaming potentials the liquid is forced through the tube and the potential difference measured. In every case it is the relative velocity of motion of the two phases in the electric field that is measured, or conversely, the intensity of the field set up when the particles are caused to move. 

The first results about foam electrokinetics have been reported by Sharovamikov [62,63]. An electroosmotic liquid transport is observed in foams from solutions of ionic surfactants (NaDoS, CTAB, PO-3A, etc.) and it is larger than in systems with solid capillaries (specific transport from 1.6-1 O 6 to 210 6 m3 C 1). The maximum electroosmotic pressure depends on the initial pressure in borders and reaches 1 Pa. The addition of dedecanol to the NaDoS solution sharply decreases the electroosmotic transport but increases the electroosmotic pressure. To reduce the influence of border and film non-homogeneity that originates in a static foam under gravity, the electrokinetic studies have been performed in an advancing foam [62]. The specific electroosmotic transport depends on the capillary pressure and reaches a maximum value at pg = 0.5 kPa. The streaming potential (up to 10 mV)... 

Streaming Potential Streaming potential is the same phenomena operating in reverse—that is, the flow of electroljdie induces an electric field, E, which is measured. Using transport equations the volumetric flow rate can be related to the pressure drop across the capillary, AjP/L, giving... 

Streaming Potential.—The velocity of a liquid flowing in a capillary tube varies with the distance x from the center o the tube, and according to Poiseuille s treatment it is equal to-P r — x )/4tj , where r is the radius and I is thcTength of the tube.. The r moving part of the double layer is at a distance r — cZ from the center (Fig. 126) and so its velocity u is given by... 

Streaming potential pressure gradient liquid plug or capillary potential difference streaming potential (difference) per unit of pressure difference s(r Vm N- ... 

When the external circuit has a high resistance, a potential difference builds up across the capillary, the streaming potential E. It leads to a counter-... 

Electro-osmosis is the counterpart of electrophoresis in that now the materials to be studied are stationary, whereas the liquid moves at a given velocity, driven by an applied field. In streaming potential measurements an applied pressure difference is the driving force in that case a potential difference is measured. In practice, two ways are open working with capillaries or with plugs. 

With plugs and capillaries a number of electrokinetlc (streaming potentials, electro-osmosis, streaming currents) and related phenomena (conductivity, permeability) can be measured, all of these requiring a different mode of operation. 

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